Heretofore, various attempts have been made to reduce sound noise of a diesel engine (in particular, sound noise caused by engine knocking (this sound noise will hereinafter be referred to simply as “knocking sound”). For example, the following Patent Document proposes a technique of calculating, as a target value of a generation time lag between respective ones of a plurality of combustion pressure waves generated, respectively, by multiple fuel injections, a time lag for enabling a pressure level in a high frequency region to be reduced by means of interference between the combustion pressure waves, and, based on this target value, controlling an interval at which the multiple fuel injections are performed (interval between the multiple fuel injections). This technique intends to achieve reduction of knocking sound by controlling the fuel injection interval to reduce frequency components of an in-cylinder pressure while targeting a specific frequency band (2.8 to 3.5 kHz). As used in this specification, the term “combustion pressure wave” means a pressure wave generated by a phenomenon that an in-cylinder pressure rapidly rises according to combustion in an internal combustion engine, wherein this pressure wave is equivalent to a result obtained by temporally differentiating a waveform of the in-cylinder pressure.